The present invention relates to a process for the production of melamine crystals at high pureness.
More specifically, the present invention relates to a process of the non-catalytic condensation type (pyrolysis) of melt urea and subsequent crystallisation through cooling with liquid ammonia of the melt melamine so produced.
In the following description and attached claims, expressions like: xe2x80x9cmelamine (crystals) at high purenessxe2x80x9d or xe2x80x9csubstantially pure melaminexe2x80x9d, are used to indicate a product with a concentration of melamine greater than 99%.
The invention also relates to a plant for carrying out the aforesaid process, as well as to a method for modernizing an existing plant for melamine production.
As known, in the field of melamine production, the need is more and more felt to provide a process suitable for producing in an easy, effective and reliable way melamine crystals at high pureness, whereby the process should imply low energy consumption as well as low investment, operational and maintenance costs for carrying it out.
In order to meet the above-mentioned need, processes for condensing melt urea to melamine in a non-catalytic way have been proposed in the field. Such processes operate at high pressure (80-150 bar) and the crystallisation procedure of the product therein involved is carried out in a number of steps.
During a first cooling step, ammonia (both gas and liquid) is injected for cooling the high purity melt melamine from the synthesis temperature to a temperature slightly higher than that of crystallisation, so to convert back into melamine possible degradation by-products (Melem, Melam, Melon, etc.) and counter the deammoniation of melamine during its crystallisation.
During a second cooling step, through the injection of an aqueous ammonia solution, the actual crystallisation of melamine is made occur.
Although this way of cooling the melt melamine allows obtaining a product having a high and constant pureness in time, it anyway requires suitable interventions to the cycle of mother liquors (ammonia aqueous solution), with ensuing high investment, operation and maintenance costs for the plant intended for carrying out the process.
In order to obviate to these drawbacks, processes for the production of high purity grade melamine have been proposed successively in the field, wherein the crystallisation of the melt melamine is carried out using only ammonia (gaseous and/or liquid), as coolant, without employing process water. In this way, the plant for melamine production is simplified and the investment and operation costs are reduced.
A process of this type is for example described in WO 98/0453.
Although advantageous as far as certain aspects are concerned, these process types are not always adapted for guaranteeing a product having a constant high purity and quality in time. Consequently, the so produced melamine may be used for limited uses.
Further on, the use of ammonia (gaseous and/or liquid) during the crystallisation of melt melamine as unique coolant, as it has been proposed in the prior art, implies high energy consumption and operation costs in order to separate and recover ammonia from the melamine crystals.
Because of these disadvantages, the processes according to the prior art do not allow obtaining a high quality product in an easy, reliable and economical way, notwithstanding the need more and more felt in the field.
The technical problem at the basis of the present invention is that of providing a process for the production of melamine crystals at high pureness which is simple, effective and reliable and at the same time does not require relevant investment, operation and maintenance costs and high energy consumption for carrying it out.
The above mentioned problems is solved, according to the invention, by a process of the above indicated type, which comprises the steps recited in the attached claim 1.
Preferred and advantageous embodiments of the present process are further recited in the attached claims 2-14.
As will be seen in the following, according to a preferred embodiment of the present invention, the melamine produced in a first reaction space is suitably stabilised upon removal of the vapours in a second reaction space. The second reaction space operates at the same pressure and temperature of the first reaction space in presence of gaseous ammonia at about 400xc2x0 C. The so-obtained melt melamine is then cooled down quickly and crystallised at the same reaction pressure by inner circulation of liquid ammonia.
In order to carry out the aforesaid process, the present invention advantageously provides a plant for the production of melamine crystals at high pureness of the type reported in the attached claims 15-21.
According to a further aspect of the present invention, there is also provided a method for the modernization in situ of a plant for melamine production, as will described in greater detail in the description hereinbelow.
The features and the advantages of the present invention will become clear from the following indicative and non-limiting description of an embodiment of the invention, made with reference to the attached drawing.